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How CRISPR Gene Editing Works


Bacterial Assassins
The CRISPR-CAS9 nuclease protein uses a guide RNA sequence to cut DNA at a complementary site.  Cas9 protein: white smoothed surface model; DNA fragments: purple & pink ladder; RNA: light green ladder.
The CRISPR-CAS9 nuclease protein uses a guide RNA sequence to cut DNA at a complementary site. Cas9 protein: white smoothed surface model; DNA fragments: purple & pink ladder; RNA: light green ladder.
MOLEKUUL/SCIENCE PHOTO LIBRARY/Getty Images

In 1987, scientists studying E. coli discovered repeated segments in the bacteria's DNA. This types of pattern in bacterial DNA is unusual, so they perked up when they noticed it, and reported the finding. Over time, scientists started seeing this pattern in many different types of bacteria, but there was still no hypothesis for what it was and why it was there. But in 2005, a search in a DNA database showed that the "clustered regularly interspaced short palindromic repeats" (or CRISPR) matched virus DNA.

But why would bacteria have harbored away virus DNA? Scientist Eugene Koonin hypothesized that when bacteria survive a virus attack, they cut up the virus into small pieces and store some of the virus DNA in their own genome so that they can later recognize and attack the virus if they happen to meet it again. They basically store a picture of the virus in their back pocket so that they'd recognize the bad guy if he were to ever show up again — a remarkable defense mechanism of the bacterial immune system.

Koonin's hypothesis was right. If that virus hits again, the bacteria manufacture special "assassins." These assassins can read the RNA sequence of any virus DNA they run into, recognize if it matches the information they've stored in their DNA, trap it and chop it up. It is as if the bacteria has created very specific, smart scissors.

This discovery was pretty cool, but not as cool as what University of California, Berkeleyscientist Jennifer Doudna thought to do with the information. She suggested that scientists could use CRISPR as a tool to help them edit genes. If they equipped the bacteria with a segment of DNA that is known to be bad — say a gene that causes blindness — they could send the bacteria in to seek out the bad gene, where the bacteria would find it and assassinate it. And then we could take advantage of the natural repair mechanism in the bacterial cells to throw a more desirable gene in its place [source: RadioLab].

It worked! And it kept working! Reversing blindness mutations has just been one of the ways CRISPR has been shown to operate. It's stopped cancer cells from multiplying, made cells impervious to HIV, helped us create disease-resistant wheat and rice, and countless other advances. In 2015, Chinese scientists even attempted to use the technology on nonviable human embryos but in only a few cases did CRISPR make the right cuts to the DNA [source: Maxmen]. Those results will probably improve over time.

But this begs the question: Do we even want to use it on embryos? Should we be allowed to? Who will regulate the use of CRISPR?